The present invention is generally directed to a hybrid optical and electronic imaging system, and more particularly to such an imaging system suitable for use, for example, as a camera viewfinder.
Hybrid cameras (i.e., cameras that capture an image both photographically on a film medium and electronically on a digital medium such as a CCD, as that term will hereinafter be used) are known in which separate or at least partially separate optical systems are used to direct light from an object scene to a film imaging plane, an electronic imaging plane, an ocular lens of a viewfinder, or some combination of these. For example, U.S. Pat. No. 4,714,962 discloses a hybrid camera which uses parallel optical systems for film and electronic capture. U.S. Pat. No. 4,742,369 discloses various hybrid cameras, some of which use a shared optical system for both film and electronic capture, and others of which use parallel optical systems. In an embodiment using parallel optical systems, the ""369 patent discloses a camera-supported display for electronically captured images. The display includes a printed line to indicate, for an unspecified range to an object to be photographed, the portion of a displayed electronic image which corresponds to an image captured on film. Commonly assigned U.S. Pat. No. 4,949,117 discloses a hybrid camera of the single lens reflex type in which the film, the electronic imaging device, and the viewfinder share an objective lens system. U.S. Pat. No. 4,978,983 discloses paired film and electronic cameras with parallel optical systems in which the field of view of the electronic imaging device is shown to overlap that of the film. The output of the electronic imaging device is processed so that only that portion of the electronically captured image corresponding to the film image is downloaded to an external display, thereby correcting on the display for parallax between the parallel optical systems.
Hybrid cameras in which a common objective lens system directs light to the film, the electronic imaging device and the viewfinder are free of parallax problems. In contrast, where parallel optical systems are used in a hybrid camera, both vertical and horizontal parallax may exist between the optical system of the viewfinder and the optical systems for film capture and electronic capture. Without appropriate provision for correction of parallax, a user of such cameras may experience difficulty composing images in the viewfinder. For example, in a digital mode the user wants to be able to tell which portion of the field of view of the viewfinder corresponds to that of the optical system for electronic capture, whereas in the film mode the user wants to know which portion corresponds to the field of view of the optical system for film capture. In the hybrid mode the user wants to be sure that the field of view of the viewfinder is such that a desired image will be captured properly both on film and by the electronic imaging device.
In a hybrid camera in which one optical system comprises the camera taking lens system (i.e., the optical train directing light to the xe2x80x9cfilm,xe2x80x9d as opposed to an electronic imaging device or to the eyepiece) and one or more separate or at least partially separate optical systems for CCD and/or eyepiece imaging, it is desirable to register the film image, the electronic image and the viewfinder image. In other words, the camera user wants the viewfinder image to be substantially identical to the captured electronic and film images, which should also be substantially identical to each other.
Accordingly, there is a need for an optical system suitable, for example, for use as a camera viewfinder that provides a visual image to the user that is the same as the image to be recorded, either digitally, on film, or both.
There is also a need for an optical system suitable, for example, for use as a camera viewfinder, that is compact in design and application, which is more economical than two separate viewfinder optical systems for film and electronic imaging, respectively, and which is environmentally stable.
The optical system described below provides these advantages and others.
The invention described herein is directed to an image forming optical system and, preferably, to such a system suitable, for example, for use as a viewfinder in a hybrid camera having a separate optical system for capturing an optical image of the object on a film medium.
An embodiment of the optical system includes an electronic imaging channel having an optical axis that forms a first image of an object in an image plane, and a visual imaging channel having an optical axis that is at least in part common with the optical axis of the electronic imaging channel (i.e., having a common optical axis segment) that forms a virtual image of the object in another image plane, adapted to form substantially identical first and second images.
Preferably, the optical system has an afocal 3xc3x97 or less zoom objective lens system located along the common optical axis segment.
In a preferred embodiment, the optical system is a viewfinder optical system for a hybrid camera. The virtual image is formed in an eyepiece of the visual imaging channel and is viewed, conventionally, by a user bringing the camera in proximity to his/her eye and viewing the object to be photographed. Alternatively, the viewfinder may be in the form of a viewing screen of known types. No photograph need be taken to view the virtual viewfinder image as this is a passive part of the system. The first image is a digital image that is electronically stored and displayed on, for example, an LCD. However, the electronic image capture does not occur until the picture is xe2x80x9ctaken,xe2x80x9d and is thus an active part of the optical system. Therefore, while the optical system described herein is a viewfinder, only the passive, visual imaging channel provides a viewfinding image; the active, electronic imaging channel provides a xe2x80x9cverificationxe2x80x9d image of the virtual image seen in the viewfinder and the analog image captured on a film medium.
In an aspect of this embodiment of the invention, the split visual and electronic channels of the optical system are provided by an nth optical element located along the common segment of the optical axis that preferably transmits light from the object for forming a digital image, and which preferably reflects light from the object for viewfinder visual imaging by the user. In a preferred aspect, the optical element is a plate beamsplitter or a beamsplitter cube. The optical system further includes an mth optical element located along the optical axis in the visual imaging channel for sizing, inverting and aiming an intermediate real image to form the virtual image in the viewfinder eyepiece. The mth optical element preferably is a three reflection prism. The entrance and exit surfaces of the prism are preferably plano, but alternatively may have power and aspheric aberration correction.
The optical system further preferably includes a CCD that receives the light of the first image, and an LCD coupled to the CCD for displaying the first image.
Preferably, the viewfinder according to the invention will find particular application in a camera, most preferably in a hybrid camera, and the front to back dimension of the viewfinder will be the limiting front to back dimension of the camera, thus providing a hybrid camera having a thickness less than 24 mm.
The optical components of the digital channel of the optical system according to the invention are preferably glass elements that provide the system with environmental stability against, for example, temperature variations in the camera operating environment. The optical components of the visual channel are preferably plastic elements that provide the system with cost advantages over glass components.
The optical system according to the invention is advantageous for a number of reasons. It provides a viewfinder system that enables an image on an LCD via a CCD, and a virtual image for viewing by the user through the eyepiece where the digital and visual images are substantially identical. Thus, two images of the xe2x80x9csamexe2x80x9d object are created. The zoom objective lens system is common to the formation of both images thus maintaining the registration of the visual and digital images. Preferably, a hybrid camera incorporating the viewfinder system according to an embodiment of the invention provides a film (analog) image that is also the same as the digital and visual images at any zoom range. Therefore it can be said that what you see is what you get. The optical system further offers compactness in design and a good performance versus cost metric.